How to get unique items using Linq, according to one of their properties

Question

Let say I have a list of objects with the following properties (FirstName, LastName):

var people = new []
{
    new { FirstName = "John", LastName = "Smith" },
    new { FirstName = "Dale", LastName = "Smith" },
    new { FirstName = "Jermey", LastName = "Ducey" },
}.ToList();

Using Linq how can I return just the object containing "Jeremy, Ducey" if I am search by unique last names.

Distinct() still returns one instance of smith, and every answer leads me to GroupBy() which only return groups properties OR Distinct() which doesn't do what I want.

Do you literally have that list of strings? Including `"First, Last"` as the first string? Or do you actually have a list of objects with properties `First` and `Last`? — Enigmativity, Nov 08 '20 at 02:09
So the key you're looking for is last name, having count > 1, distinct. I think that's why answers are leading you to GroupBy as part of the solution. — Mark C., Nov 08 '20 at 02:17
I edited the title, to differentiate this question from a [similar one](https://stackoverflow.com/questions/292307/selecting-unique-elements-from-a-list-in-c-sharp "Selecting Unique Elements From a List in C#"). — Theodor Zoulias, Nov 08 '20 at 08:44
Hey @Enigmativity! This is a nice question, and it's a pity to be closed as unclear. Could you suggest a way to improve the question so it can be reopened? — Theodor Zoulias, Nov 09 '20 at 21:51

Enigmativity · Accepted Answer · 2020-11-08T08:51:49.223

3

This does what you want:

var people = new []
{
    new { FirstName = "John", LastName = "Smith" },
    new { FirstName = "Dale", LastName = "Smith" },
    new { FirstName = "Jermey", LastName = "Ducey" },
}.ToList();

var uniqueLastNames =
    people
        .GroupBy(x => x.LastName)
        .Where(xs => xs.Count() == 1)
        .SelectMany(xs => xs)
        .ToList();

It gives me:

edited Nov 08 '20 at 08:51

answered Nov 08 '20 at 02:19

Enigmativity

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AFAIK the [`Count()`](https://referencesource.microsoft.com/system.core/system/linq/Enumerable.cs.html#41ef9e39e54d0d0b) operator is implemented with a fast path for collections, and the [`Grouping`](https://referencesource.microsoft.com/system.core/system/linq/Enumerable.cs.html#7bb231e0604c79e3) is an `IList`, so the `xs.Count() == 1` should be quite efficient. – Theodor Zoulias Nov 08 '20 at 07:33
1

@TheodorZoulias - Yes, that checks out. :-) – Enigmativity Nov 08 '20 at 08:51

How to get unique items using Linq, according to one of their properties

1 Answers1